The present invention relates in general to integrated circuits, and in particular to voltage reference circuitry with power supply (VCC) compensation to provide a single voltage level output across commercial, industrial and military VCC specification ranges.
In many circuit applications it is desirable to provide a stable reference signal that is insensitive to variations in power supply voltage level as well as temperature and process variations. An exemplary circuit application for a stable reference voltage can be found in memory circuits that use sense amplifiers for detecting the logic value stored in a selected memory cell. One type of sense amplifier used in this context has a region of operation where the circuit is considered stable with on-chip noise. Stability is a concern both when reading a logic high level and when reading (i.e., sensing) a logic low level. When reading a high logic level, the reference voltage should be high enough to have the nodal voltages within the sense amplifier read a logic high strongly. On the other hand, when reading a logic low level, the reference voltage should not be too high because that would prevent the nodal voltages within the sense amplifier from reading a logic low strongly.
With these conflicting reference voltage requirements, having a circuit that can produce a voltage which satisfies both conditions requires that the reference voltage be at a level between the two extremes of sense amplifier reference voltage needs. The variations in the power supply voltage VCC make this task difficult. Typical prior art circuits generate a reference voltage with a strong dependency on VCC levels, wherein the reference voltage is lower for low VCC and higher for high VCC. This makes the sense amplifier less stable when reading a high logic level (e.g., an off memory cell) at lower VCC and reading a low logic level (e.g., an on memory cell) at higher VCC. Adding processing variation to this equation makes the effects more severe.
It is therefore desirable to provide a voltage reference circuit that is insensitive to variations in power supply voltage level.